During the last two decades, thermoplastic polyurethanes have gained increasing attention. The materials combine the excellent properties of urethanes with the processing convenience of thermoplastic materials. Thermoplastic polyurethanes are generally of the (AB).sub.n type, where flexible polyester or polyether segments (A) alternate with high melting polyurethane blocks (B).
Polyesterglycols (e.g., poly-1,4-butyleneadipate or polycaprolactone) or polytetramethylene ether glycols are commonly used as the flexible segments, while low molecular weight glycols (e.g., 1,4-butanediol or ethylene glycol) are most often chosen for the in situ formation of the rigid blocks of the alternating copolymer. A great variety of diisocyanates can be utilized in the preparation of these polymers, however, for reasons of commercial availability and polymer performance, 4,4'-diphenyl methane diisocyanate (MDI) has been widely preferred.
Polyesterurethanes have the disadvantage of inherent hydrolytic sensitivity, while polytetrahydrofuran type materials are of relatively high cost.
Linear urethane polymers of the above type based on low cost polypropylene ether glycols have not been prepared with satisfactory properties due to the fact that about the maximum molecular weight of commercially available polypropylene glycols made with alkali catalysts is about 3,000 for average functionality approaching 2. Even at this 3,000 M.W. limit for the polypropylene ether glycol the loss in end group functionality (--OH groups) becomes too large, causing low weights in the resulting linear polyurethane.
Accordingly, it is an object of the present invention to overcome the difficulties alluded to hereinabove and provide a method for making a thermoplastic linear polyurethane from polypropylene ether glycol having improved properties. These and other objects and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description and working examples.